The economic disposal of waste solids and the recovery of clean water from aqueous wastes is a recognized problem. Ideally, apparatuses and processes for the recovery of water from solutions of waste solids should provide ease of disposition, avoidance of pollution, economic operation, and hygienic handling and should, in addition, yield clean water. Furthermore, in the course of recovering clean water it is desirable to obtain other products or materials which are either valuable in themselves or may be utilized to further the economics of the process.
In one of the customary processes known to the prior art, water solutions of waste solids such as aqueous industrial wastes and brackish or salt waters are subjected to evaporation with subsequent condensation of the water vapor to thereby recover water free of the waste solids.
In many cases of direct evaporation, particularly direct multiple-effect evaporation, scaling or fouling of the evaporator tubes results. One measure taken to prevent evaporator scaling is the use of antiscaling compounds such as zeolites to remove dissolved minerals. However, industrial wastes, sewage and brackish and salt waters have such high levels of dissolved solids that the quantities of antiscaling compounds required make this procedure prohibitively expensive, and further result in disposal problems of the added chemicals.
As an alternative method to prevent evaporator scaling, evaporator manufacturers have resorted to separate tube nests which can be brought into operation as required so that the scaled or fouled evaporator units may be removed from the line and cleaned to remove the material that has caused the fouling or scaling. This changeover, which results in the loss of time and also results in disturbing the equilibrium of the evaporation system, requires operator attention and a considerable number of switching valves which may be operated either automatically or manually. Thus there is added capital cost due to the control system as well as the additional tube bundles required.
The materials which cause fouling and scaling of evaporator tubes are often corrosive in nature. Thus special alloys such as stainless steel and other high performance metals are often required for tube materials. This is yet another factor resulting in increased capital cost.